Definition of law of cosines

1: a law in trigonometry: the square of a side of a plane triangle equals the sum of the squares of the remaining sides minus twice the product of those sides and the cosine of the angle between them

2: a law in trigonometry: the cosine of an arc of a spherical triangle equals the product of the cosines of the remaining arcs plus the product of the sines of those arcs and the cosine of the angle between them